The invention relates to a hydraulic pressure fluid control system for controlling the displacements of a double-acting servo-motor having preferably equal hydraulic cross sections, particularly for servo-assisted steering gear in motor vehicles, comprising a control valve arrangement which in neutral position cuts off the working chambers of the servo-motor from a pressure source and a return and the pressure source from the return, and which contains valve members in the form of two control pistons of which each is associated with one working chamber; actuating means for manually operating the control pistons in dependence upon relative motion between an input member and an output member of the pressure fluid control system, the output member being connected to the working member of the servo-motor, e.g., to its servo-piston, and the movements of the working member being retransmitted to the control valve system by mechanical feedback means; the control piston being provided with reaction faces to which the pressures existing in the working chambers of the servo-motor are applied, so that each working chamber is associated with such a reaction face; there being further provided between each of the control pistons and the actuating member coil springs opposing the thrust of the pressure acting on the reaction faces, said springs causing a predetermined minimum pressure to act on the reaction faces and to be maintained in the working chambers, and the control pistons being so disposed in relation to the input member that starting from the neutral position of the input member the rigid coupling connection between the control piston and the input member is not achieved until a predetermined relative motion has taken place between the input member and the output member which are connected by interposed biased elastically yielding coupling means.
As known, the purpose of such hydraulic pressure fluid control systems is to provide servo-assistance and thereby to reinforce a manually applied input force or torque. If the elastically yielding coupling means between the input member and the output member are biased, this bias creates a response threshold below which the applied force will be transmitted without hydraulic servo-assistance, whereas above the threshold the previously effective transmission of force will be hydraulically reinforced. Below the response threshold the coil springs between the control pistons and the input member allow movements of the control pistons to take place, which result from changes in the pressures acting on the reaction faces due to the mechanically effected displacement of the working member of the servo-motor. They also serve to maintain a predetermined minimum pressure in the working chambers even before the input member has been operated. This minimum pressure has the effect of hydraulically clamping the servo-motor between opposing pressures and of thereby reducing shock loads and jolts while additionally eliminating noise which may otherwise be caused by cavitation or air desorption phenomena.
Such a fluid pressure control system has already been described in the U.S. Pat. application Ser. No. 427,131. The biased elastic coupling means in this instance is an annular spring. The drawback of such an annular spring is that it requires additional space although this may be small, that it is rather expensive, and that it requires carefully matched support and actuating means which are therefore difficult to provide as well as being liable to rapid wear.